Polar, hydrogen-bonded water chains with
O-H(d+)and(d–)O-H run parallel to the crystal
length (Fig. 3A); hence, fracture occurring
via O-H···O hydrogen bond rupture leaves
opposite dipoles of the molecules on freshly
created 00ðÞ 1 and (001) surfaces. Although
piezoelectric materials generate electrical sur-
face charges under mechanical stress ( 18 – 20 ),
the long-lasting remnant potential could be due
to the presence of some fatigue or permanent
strain in fractured crystals of 1. Some degree
of plasticity in 1 near the fracture tip is not
surprising given the presence of dispersive
interaction zones in its crystal packing (Fig. 2,
AandB)( 21 , 22 ).
By contrast, the neatly healed crystals showed
no such electrostatic potential gradient across
the healed crack line on the (100)/(010) faces
(fig. S15). Hence, our KPFM results are in sup-
port of the large amount of electrostatic poten-
tial generated in the case of fractured pieces,
which acts as a natural restoring force between
the broken pieces, driving the attractive re-
combination. The recombination quenches thecharges in the self-healed piezoelectric crys-
tals of 1.
Because crystalline integrity affects almost
all the physical properties of molecular crys-
tals, we used microfocus SCXRD to examine
the self-healing efficiency of 1 with respect
to internal order and alignment of rejoined
pieces. The neatly healed crystals produced
sharp diffraction peaks comparable to those of
the pristine crystals (Fig. 3, B to D, and figs.
S16 and S17). We found no split peaks even at
higher resolution (Fig. 3C). Further, completeSCIENCEsciencemag.org 16 JULY 2021•VOL 373 ISSUE 6552 325
ACDFGIB E Hdistance (μm) distance (μm) distance (μm)d
L
d
L
d
L0 50 1000.10.080.050.02Fig. 4. Quantitative assessment of structural order using Mueller matrix
analysis.(A,D, andG) Linear diattenuation parameterdL. Scale bars,
25 mm. Insets: Schematic morphology representation and optical images of
the respective crystals. (B,E, andH) Spatial variation plots ofdLcorresponding
to pristine, neatly healed, and imperfectly healed crystals, respectively.
(CandF) The microscopic orientation of the retarder axis of the pristine
crystal (C) and the neatly healed crystal (F), showing uniformity along the
crystal. Scale bars, 12mm. The slight offset in the orientation of the retarder
axis (arrow orientation) is because of a slight difference in orientation
of the crystals aligned manually on the sample stage. (I) The microscopic
orientation of the retarder for imperfectly healed crystals, which is uniform
along the crystal axis, except at the crack junction (marked by dashed
rectangular box). Scale bar, 6mm. See figs. S18 to S22 and tables S6 to S8
for the full datasets.RESEARCH | REPORTS